Replace 5 ton with a 4 ton AC; keep existing duct work

I have to install a new air conditioning unit. The old one is dead (Heil 60LC-000087CR; a 5 ton machine manufactured in 1991)

Two contractors have bid to replace it (Trane 13 SEER and a Lennox 14 SEER). Both have recommended 5 ton machines.

However on this site and other literature - proper sizing is critical; not too big, not too small. I worked through a heat load estimate on Heatload.com (37,175 BTU, 1043 CFM) and a worksheet in a book called "HVAC Fundamentals" (43,932 BTU). Both analysis took into account room area, volume, windows/type, orientation, insulation, and location - story level/over uninsulated rooms, etc. Both analysis gave a result that would indicate a 4 ton machine.

The contractors will conduct a Manual J analysis for $400.

Knowing a little bit of knowledge is a dangerous thing, I asked the contractors their opinion of going to a smaller machine.

They are concerned that the existing duct work was sized for a 5 ton machine. Without proper duct size and dampers the air flow will be reduced (1,600 cfm for the 4 ton and 2,000 cfm for the 5 ton), hot/cold spots will be felt. However the ducts work fine for the furnace (Trane XR80).

I should note that I have taped up most of the ducts joints to reduce the leakage I found when I purchased the house (built in 1953, do not know the age of the duct work - assumption is it was installed in 1991 but it looks older.) We are also adding insulation to the attic where there is none now.

I have to install a new air conditioning unit. The old one is dead (Heil 60LC-000087CR; a 5 ton machine manufactured in 1991)

Two contractors have bid to replace it (Trane 13 SEER and a Lennox 14 SEER). Both have recommended 5 ton machines.

However on this site and other literature - proper sizing is critical; not too big, not too small. I worked through a heat load estimate on Heatload.com (37,175 BTU, 1043 CFM) and a worksheet in a book called "HVAC Fundamentals" (43,932 BTU). Both analysis took into account room area, volume, windows/type, orientation, insulation, and location - story level/over uninsulated rooms, etc. Both analysis gave a result that would indicate a 4 ton machine.

The contractors will conduct a Manual J analysis for $400.

Knowing a little bit of knowledge is a dangerous thing, I asked the contractors their opinion of going to a smaller machine.

They are concerned that the existing duct work was sized for a 5 ton machine. Without proper duct size and dampers the air flow will be reduced (1,600 cfm for the 4 ton and 2,000 cfm for the 5 ton), hot/cold spots will be felt. However the ducts work fine for the furnace (Trane XR80).

I should note that I have taped up most of the ducts joints to reduce the leakage I found when I purchased the house (built in 1953, do not know the age of the duct work - assumption is it was installed in 1991 but it looks older.) We are also adding insulation to the attic where there is none now.

So the question is - 4 ton or 5 ton?

Thanks very much.

Let the dealer do a manual J then accept there findings. It might be the best $400.00 you will ever spend. This way if the unit is to small or to large you have all the proof you will need to force them to fix the situation and of course all the documents needed to take to a lawyer to really force them to fix it correctly.

You can put 4 ton into a 5 ton duct system but not 5 ton into a 4 ton. If the return air is oversize that is a super bonus because most are under sized.

HVAC1000
Thank you very much for the instructive answer - the chart is especially helpful in both analysis and discussions with the contractor. Will help us speak the same language.

Just to confirm - have the contractor do a Manual J evaluation. Based on their analysis install either the 4 or 5 ton unit while using the existing duct work (replacing the ductwork would be cost prohibitive). The 4 ton machine could use the existing duct work (assuming it was sized for the 5 ton unit)

Based on their analysis install either the 4 or 5 ton unit while using the existing duct work

YES

The 4 ton machine could use the existing duct work (assuming it was ORIGINALLY sized for the 5 ton unit)

YES

While size is important your comfort is more important. When you spend the big $$$ you want to make sure that when it gets hot the temperature and moisture level is within specification.

You never mentioned where you live BUT you might consider a Heat Pump/ A/C unit. Fuel prices are never going to do anything but go up and it is a good way to save some $$ on your heat. Naturally a heat pump unit is a bit more expensive but the pay back time should be short. You might get a price difference between a HP and a strait A/C.

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